1 . Field of the Invention
The invention pertains generally to medium-access collision-avoidance protocols, and more specifically to a receiver-initiated channel-hopping protocol which does not require carrier-sensing or code assignments by the network nodes.
2. Description of the Background Art
Medium-access control (MAC) protocols based on collision avoidance have recently received considerable attention due largely to their simplicity when utilized within wireless LANs and ad-hoc networks. In a traditional collision-avoidance protocol, a node that has data to be transmitted to a receiver, first sends a request-to-send (RTS) control packet to the receiver, to which the receiver responds with a clear-to-send (CTS) control packet if it has correctly received the RTS control packet. It will be appreciated that the sender may only transmit a data packet after successfully receiving the CTS control packet. A number of variations of this method have been developed since the proposal of split-channel reservation multiple access SRMA, which include MACA, MACAW, IEEE 802.11, and FAMA. Industry researchers have relied upon sensing the channel before sending their RTS packets, within single channel networks, to avoid data packet collisions with other packets at the intended receivers. More recently, receiver-initiated collision-avoidance protocols have also been proposed for single-channel networks, in which the receiver initiates the collision-avoidance handshake. It will be appreciated that these receiver-initiated collision-avoidance protocols also require carrier-sensing to ensure correct collision avoidance.
Requiring collision-avoidance MAC protocols on single-channel networks to sense the channel as an integral part of the collision-avoidance handshake generally limits their applicability. For example, some commercial radios do not provide true carrier-sensing, and direct sequence spread-spectrum (DSSS) radios may capture only one of multiple overlapping transmissions in a non-deterministic manner, depending on the proximity and transmission power of the sources. Even if frequency-hopping spread-spectrum (FHSS) radios are utilized, carrier-sensing adds to the complexity of the radio, which must already provide coarse time synchronization at the dwell-time level. However, the use of one or more busy-tones, to indicate when a receiver is busy, essentially requires a second transceiver, and is therefore not generally considered economically attractive.
Several proposed MAC protocols were directed at taking advantage of spreading codes to provide multiple access. These MAC protocols have been variously analyzed, such as analyzing spreading-code protocols that are sender, receiver, or sender-receiver based wherein codes are assigned to senders, receivers, or combinations thereof. The use of carrier-sensing can improve the efficiency of the network by reducing the number of unsuccessful transmissions and the amount of unwanted interference, when utilized within a receiver-based asynchronous transmission protocol. Other protocols such as a receiver-based handshake protocol for CDMA (code division multiple access) networks have been proposed. Proposals that have been put forth to implement correct collision-avoidance in multihop wireless networks, without requiring nodes to use carrier-sensing, which rely on the use of code assignments to senders, or to receivers, for eliminating the need to use carrier-sensing.
A primary limitation of protocols based on code assignments arises from the way in which senders and receivers are required to determine the codes that each other are utilizing as a pre-condition of communication. A currently-available commercial DSSS radio typically requires about eleven integrated circuits per bit, and as a result CDMA is not an attractive option. Future DSSS radios are expected to utilize about fifteen integrated circuits, thereby allowing two different systems to operate over the same DS frequency channels as were defined in IEEE 802.11, however, up to twenty six FHSS radios can be collocated. According to the FCC regulations, up to fifteen FHSS radios can be collocated with minimum interference problems. It will be appreciated, therefore, that in ad-hoc networks implemented utilizing commercial radios operating within ISM bands, that the use of code assignments does not guarantee that receivers can capture one of multiple simultaneous transmissions, and that slow frequency hopping with one or more packets sent per hop is the viable way to achieve multiple orthogonal channels in the ISM bands.
It will be appreciated, therefore, that current medium-access control (MAC) protocols for wireless networks are based on collision-avoidance handshakes between sender and receiver which either require carrier-sensing or the assignment of unique codes to nodes to ensure that intended receivers hear data packets without interference from hidden sources.
Therefore, a need exists for a MAC protocol that can take advantage of the characteristics of FHSS radios operating in ISM bands while assuring that transmissions are free of collisions due to hidden terminal interference. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed MAC protocols.